Valves of the above type are well known. Such valves are used in the first place in applications where high cleanness is required. Such applications can be various fluid engineering processes or process steps, for example for pharmaceutical and food technology purposes or other types of chemical purposes. Many fluid products are expensive and/or susceptible to contamination or dangerous in case of leakage, which means that leakage can result in an unhealthy environment for the user of a product or an environment. Tightness of a valve of the type stated above thus is crucial.
In valves of this type, a diaphragm element is operated between an open and a closed position. Fluid flowing in the valve housing can thus be easily and safely handled in an adjustable manner. The operation of the valve can take place in various ways, for example using manual or pneumatic control.
Furthermore, valves of this type should in their open position have good flow capacity without the valve being a substantial obstacle, thereby reducing the risk of the valve accumulating contaminants, such as dirt and deposits. The valve should also be absolutely tight. This condition may sometimes be difficult to achieve, for instance when using aggressive chemicals. In such cases, the sealing surfaces of the valve can be subjected to wear which results in, for instance, the material aging, becoming brittle and losing its elasticity, which may cause cracks and/or yielding and, later on, leakage.
There are thus many requirements that should be met by this type of valve, for instance good operability, good flow capacity, good tightness, sufficient cleanness.
In addition to these requirements, there are a number of ranges that should also be improved. These ranges of problems involve the following points                that the valve ensures good sealing in spite of great variations in pressure and temperature of the fluid,        that the valve ensures good sealing in spite of any plastic deformation,        that the valve has good repeatability of sealing pressure when mounting the closure element.        
In sealing, an elastic packing is usually compressed by prestressing a bolt. However, problems may arise after a while when the elastic material ages or is plastically deformed, which may result in leakage.